Metals are often coated with paint or another coating material to prevent corrosion. If the coating is scratched or otherwise removed from an area, however, the underlying metal surface could be exposed to a corrosive environment. To solve this problem, inhibitor-enhanced coatings have been developed. Corrosion inhibitors are released from the inhibitor-enhanced coatings upon the occurrence of a triggering event, such as a change in pH. If the surrounding environmental conditions are corrosive, the pH changes after the paint or other coating is scratched or otherwise removed.
Inorganic corrosion inhibitors include chromates, phosphates, molybdates, and nitrites. One of the main disadvantages of inorganic inhibitors is their toxicity. In fact, some inorganic inhibitors have been proven to cause diseases.
Benzotriazole and its derivatives are some of the most effective corrosion inhibitors for the protection of metals, especially copper and transition metals. These corrosion-inhibitors are not toxic like the inorganic inhibitors discussed above. The corrosion-inhibiting performance of benzotriazole in some environments, e.g. seawater, is not alone sufficient to prevent the corrosion of metals. Instead, benzotriazole must be combined with a passive protection mechanism, e.g. paint coating.
Because benzotriazole is partially water-soluble, it can leach out from the coating upon exposure to the water. The direct combination of benzotriazole and paint may form voids in the paint coating layer. These voids diminish the protective qualities of the paint layer. Increased anticorrosion performance is achieved by placing benzotriazole within nano- or microscale encapsulating systems and adding the benzotriazole-containing encapsulating system into the paint. The nano- or microscale encapsulating systems include polyelectrolyte and polymer microcapsules, sol-gel nanoparticles, porous silica, and nanotubes.
Prior research has explored the storage of benzotriazole within halloysite clay tubules and the addition of the benzotriazole-loaded halloysite tubules into paint. Ordinarily, benzotriazole is quickly released from the halloysite tubules, but a sustained release of benzotriazole is desirable for prolonged corrosion inhibition.
U.S. Patent Application Publication No. 2009/0078153 to Shchukin et al. (incorporated herein by reference) describes a process of loading a solid substrate (e.g., metal nanoparticles, metal oxide nanoparticles, metal oxide nanotubes, carbon nanotubes, or halloysite nanotubes) with a corrosion inhibitor (e.g., quinaldic acid or mercaptobenzotriazole); coating the solid substrate with a polymer or polyelectrolyte shell using the layer-by-layer deposition technique; and adding the coated nanoreservoir into paint. The polymer or polyelectrolyte shell prevents the release of the corrosion inhibitor from the nanotubes until the polymer or polyelectrolyte shell is triggered by an event to which the particular polymer or polyelectrolyte shell is sensitive (e.g., change of pH, ionic strength, temperature, humidity, light, or mechanical stress). However, the layer-by-layer deposition technique does not lend itself to large-scale manufacturing. Also, the loading efficiency of these capsules are low and often not sufficient for long-term corrosion protection.